Horizontal wells have forced a reconciliation with the concept of permeability anisotropy: vertical-to-horizontal and horizontal-to-horizontal. The former, if favorable, may indicate whether a horizontal well is attractive compared to a vertical well; the latter is important in well orientation. While other authors and we have postulated at least a qualitative relationship between stress and permeability orientation and we have suggested that wells should be oriented normal to the maximum stress/permeability orientation, it is often neither easy nor possible to measure stresses. In fact substantial permeability anisotropy has been found even in formations where stresses approach isotropy. In unconsolidated or loosely consolidated formations stress measurements may not even be plausible. Productivities deviate substantially among wells in the same reservoir and the well orientation has been considered as a major reason.

There is, therefore, a compelling rationale to detect and measure permeability anisotropy. Ideally this should be done before a horizontal well program is initiated in a reservoir, but if this is not possible, shortly thereafter. Both vertical and horizontal pressure interference testing have been used for permeability determinations. Under the assumption that the geometrical mean permeability, k¯, is available from a vertical well test, other authors have suggested that tests in two horizontal wells, forming an angle between them, can provide the values of the two principal horizontal permeabilities. We are suggesting a further development requiring only the linear flow regime in three arbitrarily-oriented horizontal wells. A field case study illustrates the approach. Using the analysis results, the optimal well trajectory is determined, and economics of the misorientation are computed.

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